This application claims priority to Korean Patent Application No. 2004-63857, filed on Aug. 13, 2004 in the Korean Intellectual Property Office, the disclosure of which is hereby incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates generally to wireless communication, and more particularly to a method and system for transmitting and receiving data frames with boosted power for data fields of a predetermined data type such as for channel estimation preambles.
2. Description of the Related Art
In a MIMO (multiple input multiple output) communication system, data is communicated at high speed without increasing bandwidth by simultaneously using a plurality of transmitter antennas and receiver antennas. The transmitter antennas simultaneously transmit different data through channels. Such transmitted signals are mixed and received in each of the receiver antennas of a receiver that separates the transmitted signals using channel estimation.
FIG. 1 is a block diagram illustrating a general MIMO communication system. Referring to FIG. 1, the MIMO communication system includes a transmitter 100 with three transmitter antennas 102, 104 and 106 and a receiver 110 with four receiver antennas 112, 114, 116 and 118. Such numbers of the transmitter antennas and receiver antennas is by way of example only.
Further referring to FIG. 1, signals communicated in the MIMO communication system are transformed by transmission channel characteristics between the transmitter antennas 102, 104 and 105, and the receiver antennas 112, 114, 116 and 118. In addition, noise is also added to the transformed signals. Subsequently, the signals are received in the receiver.
Assume that signals transmitted by each of the transmitter antennas 102, 104 and 106 are x1, x2 and x3, respectively. In addition, assume that signals received by each of the receiver antennas 112, 114, 116 and 118 are y1, y2, y3 and y4, respectively. In addition, assume that the channel characteristic between one of the transmitter antennas 102, 104 and 106 and one of the receiver antennas 112, 114, 116 and 118 is Hnm (n being a transmitter antenna number, n=1, 2 or 3; and m being a receiver antenna number, m=1, 2, 3 or 4). In that case, relationships between signals transmitted by the transmitter antennas 102, 104 and 105 and signals received by the receiver antennas 112, 114, 116 and 118 are expressed as follows in mathematical equations 1:y1=H11·x1+H21·x2+H31·x3+noisey2=H12·x1+H22·x2+H33·x3+noisey3=H13·x1+H23·x2+H33·x3+noisey4=H14·x1+H24·x2+H34·x3+noise  [Equations 1]
Thus, the signals, y1, y2, y3 and y4 received by the receiver antennas 112, 114, 116, and 118, respectively, are each comprised of a combination of signals x1, x2, and x3 transmitted from the transmitter antennas 102, 104, and 106, respectively. Each of the signals y1, y2, y3 and y4 is measurable by the receiver 110. If the noise is removed from Equations 1, the channel characteristics Hnm may be determined from the first order Equations 1 with x1, x2, x3, y1, y2, y3 and y4 being known.
In addition, even with the noise, a minimum mean square error (MMSE) using a matrix may be determined such that the transmitted signals x1, x2 and x3 are determined from the measured signals y1, y2, y3 and y4. In any case, the estimation of the channel characteristics (i.e., channel coefficients) Hnm is essential for performance of the MIMO communication system of FIG. 1.
Furthermore, data frames are transmitted and received in the MIMO communication system of FIG. 1 according to an OFDM (orthogonal frequency division multiplexing) format of the IEEE 802.11a standard. In such a standard, channel estimation is performed using a preamble transmitted as part of a data frame for each of the transmitter antennas.
FIG. 2 shows data frames according to the OFDM format transmitted by each of the transmitter antennas 102, 104, and 106 (Tx_ANT1, Tx_ANT2 and Tx_ANT3, respectively) of FIG. 1. Referring to FIG. 2, each data frame includes a preamble field including a short preamble and a long preamble, a header field, and a payload field.
Each of the short and long preambles includes a plurality of iterative training sequences, according to the IEEE 802.11a standard. Thus, the short preamble includes short training sequences that are iterated ten times, and the long preamble includes long training sequences that are iterated twice.
The short preamble is used for AGC (automatic gain control) convergence, timing synchronization, and coarse frequency synchronization in the receiver. The long preamble is used for channel estimation and fine frequency synchronization in the receiver. Thus, the long preamble is also referred to hereafter as a “channel estimation preamble”. The header field includes signal field information (RATE, LENGTH) used to decode data in the payload field which includes the transmitted data. Such data fields individually are known to one of ordinary skill in the art.
Further referring to FIG. 2, the transmitter antennas (Tx_ANT1, Tx_ANT2 and Tx_ANT3) simultaneously transmit different data (Data1, Data2, and Data3). On the other hand, the transmitter antennas (Tx_ANT1, Tx_ANT2 and Tx_ANT3) transmit the channel estimation preambles (long1, long2 and long3) with time orthogonality. Thus, time lags (null time periods) are used such that transmission of the channel estimation preambles (long1, long2 and long3) do not overlap. For example, when a channel estimation preamble (long1) is transmitted by the first transmitter antenna 102 (Tx_ANT1), the second and third transmitter antennas 104 and 106 (Tx_ANT2 and Tx_ANT3) are in a null time period with no data transmission.
FIG. 3 illustrates signal powers for the data frames of FIG. 2. Generally, the transmitter 100 of FIG. 1 has limited total available power. In the MIMO communication system of FIG. 1 of the prior art, each of the transmitter antennas (Tx_ANT1, Tx_ANT2 and Tx_ANT3) uses a divided power that is the total available power of the transmitter 100 divided by the number of the transmitter antennas 102, 104, and 106.
Referring to FIGS. 2 and 3, since the channel estimation preambles (long1, long2 and long3) are sent with time orthogonality, the transmission power used in one of the transmitter antennas 102, 104, and 106 for transmitting the channel estimation preamble is only one-third of the total available power of the transmitter 100. Thus, two-thirds of the total available power of the transmitter 100 is wasted during transmission of the channel estimation preambles (long1, long2 and long3) in the prior art.